The invention relates to a method for measuring the displacement of a wheel of a vehicle relative to the frame thereof or relative to a fixed point of reference, when this vehicle is located on a test stand, in which the vehicle, preferably via the said wheel is loaded, statically or dynamically.
According to the present state of the art, such measurements are taken using linear inductive wheel motion recorders, laser projection sensors or milometers on the basis of unwinding reels of cord in conjunction with a rotary encoder.
All these commonly-used measuring systems and the test methods have the drawback that only a limited excitation of the wheel can be measured. Hence, on account of the technical features of the existing measuring systems, only a maximum rotation or inclination of the wheel in the region of 20 degrees can be measured. In addition, these existing systems are no good for taking measurements if and when the wheel is loaded at a frequency which is higher than 10 Hz or if this wheel is subjected to large excitations.
These existing test set-ups are extremely complex in their design, hard to calibrate and it is, on top of that, very laborious to build in additional points of reference on the frame.
The aim of the method, according to the invention, is to remove these drawbacks and to propose a method which allows measurements to be taken at very high frequencies and with very large displacements of the wheel relative to the frame. With this method it is also possible to measure very large rotations or inclinations of the wheel.
In addition the method, according to the invention, offers the added advantage that, in a straightforward way, a high number of extra points of reference can be incorporated in the frame. These points of reference can be depicted in a shared system of coordinates relative to one and the same coordinate system.
To this aim, in the method according to the invention, in a first step, the position of at least three, not lying in a straight line, points of reference on the said wheel are measured by means of a measuring system which, preferably, comprises linear cameras or matrix cameras, in which subsequently, in a second step, the said relative displacement is calculated.
To this end, prior to the first step, the said wheel is subjected to a revolution about the axis of rotation thereof while successive positions of at least one of the points of reference are measured on the thus, through the point of reference under consideration, traced arc, in which, on the one hand, a plane is defined which is parallel to this arc and, on the other hand, the centre of the circle on which the arc lies and which lies on the said axis of rotation is calculated, so as to determine a coordinate system whose first axis coincides with the axis of rotation and a second and third axis lie in the said plane.
In a preferred embodiment of the method, according to the invention, the said coordinate system is displaced along the direction of the axis of rotation over a known distance such that the said plane coincides with the median transverse vertical plane of the wheel.
According to a special embodiment of the method, according to the invention, the position of the points of reference of several wheels of the said vehicle in a state of rest is determined relative to one another such that the positions of these points of reference can be expressed in terms of one and the same coordinate system.
Other peculiarities and advantages of the invention will soon become apparent from the following description of a specific embodiment of the method and the configuration according to the invention; this description is only given as an example and does not limit the scope of the protection that is being claimed; the reference numbers used hereinafter bear upon the figures appended hereto.